Introduction
Recirculating aquaculture systems (RAS) provide possibilities to rear fish in a highly controlled environment. Some of the advantages of RAS-based farming include minimum water use , improved biosecurity, efficient control of production parameters, better protection from challenging environmental conditions , among many others. All these contribute to ensuring an ideal environment that fosters health, welfare and performance. Disinfection strategies play a crucial part in maintaining optimal water quality and preventing disease outbreaks. These protocols are ada pted to different levels of farm operations – from materials and ancillary equipment used daily, to intake and RAS loop water and to system-wide disinfection after each production cycle. This paper summarises different aspects of disinfection in both freshwater and brackish water RAS of Atlantic salmon aquaculture. This study discusses the effects of disinfection on both the fish and the RAS environment.
Materials and methods
Four studies are included in this paper, each covering different aspects of disinfection in RAS . Sub-study 1 surveyed different salmon RAS farms in Norway and North America to benchmark the current disinfection protocols. Sub-study 2 established the thresholds and biological consequences in post-smolt of the use of ozone in brackish water RAS. Sub-study 3 explored the health and welfare aspects of using peracetic acid as a routine disinfectant in RAS. Lastly, Sub-study 4 explored biosecurity breach in RAS and system disinfection following an outbreak. Yersinia ruckeri was used as a model pathogen.
Results
Sub-study 1: Twenty-five (25) salmon RAS farms participated in total. The survey highlighted that: 1) despite having disinfection protocols in-house, majority of which were not experimentally verified; 2) Norway and North America differed on the disinfectants commonly used; 3) efficacy and safety were the common criteria for selecting the disinfectant in both regions.
Sub-study 2: A 10-day exposure trial identified that the range 300-350 mV was the safe range for ozone use in salmon brackish water. Higher than 350mV resulted in substantial mortality and health issues. The long-term trial using the identified safe dose revealed that ozone use had minor effects on survival, external welfare and production performance. It had a favourable impact o n gill health. Ozone exposure did not alter the ability of salmon to respond to the secondary stressor.
Sub-study 3: In the brackish water experiment , we found that daily PAA dosing triggered a slight local and systemic oxidative stress. Pathological alterations were predominant in the gills, where cases of epithelial lifting, hypertrophy and clubbing were prevalent. Lastly, oxidant exposure did not alter the ability of salmon to mount robust physiological stress responses to a secondary stressor. In the freshwater trial, continuous and pulse application resulted in the modulation of antioxidant defence genes in the mucosal organs. Histology revealed minimal changes in the key structures of the skin, gills and olfactory organ.
Sub-study 4: The biosecurity breach simulation studies found that the breach through intake water resulted in higher mortality than introducing a n infected fish to the system. Mortality was similar whether biosecurity was breached via the intake water once or for 3 successive days. Diseased fish developed classical external signs of yersiniosis, including skin darkening, exophthalmia, and haemorrhaging. Disinfection of the system using pH manipulation following a Yersinia outbreak was effective.
Conclusions
Disinfection is an integral component of a RAS facility. This series of studies revealed that disinfection strategies varied between regions, disinfection of the RAS loop water was crucial to maintain optimal water quality but could impact fish health and welfare when not appropriately managed , and biosecurity breach simulations offered insights into the dynamics of infection in RAS. These results are expected to contribute to developing a system-wide disinfection protocol in salmon R AS.